In electrophotography, after a surface of a photoconductor drum is charged by corona discharge or the like, the charged surface of the photoconductor drum is exposed to a laser or the like to form an electrostatic latent image, which is then developed by a toner to form a toner image, and further the toner image is transferred onto a recording medium to obtain an image with high quality, in general. The toners which may be employed in such a development process are usually provided by: mixing a binder resin such as a thermoplastic resin with a colorant, a charge control agent, a release agent, etc.; kneading the mixture; pulverizing and classifying to give toner particles having an average particle diameter of 5 to 10 μm. Furthermore, for the purpose of imparting flowability to the toner, controlling charge of the toner, and improving easiness of cleaning of the toner from the photoconductor drum, inorganic fine powder such as silica, titanium oxide or alumina is externally added to the toner.
However, these inorganic fine powders tend to be negatively charged in general, and in particular, silica exhibits strong negatively charging property. Thus, in the case in which these inorganic fine powders are used in positively chargeable toners, inorganic fine powders having a positively chargeable polar group introduced on the surface thereof are used.
Specifically, provided are: a toner to which a silica fine powder was externally added which had been subjected to a surface treatment with an aminosilane coupling agent or the like having a positively chargeable polar group; a toner to which a silica fine powder was externally added which had been subjected to a surface treatment with a positively chargeable charge control agent, and a hydrophobilizing agent; and a toner to which a silica fine powder was externally added which had been subjected to a surface treatment with a silane coupling agent having a functional group containing a nitrogen atom, and a silicone oil having a side chain containing a nitrogen atom. These toners exhibit strong positively charging property due to having a positively chargeable polar group such as an amino group on the surface of the toner (i.e., the surface of silica fine powders).
In addition, a toner to which a silica fine powder was externally added was proposed. The silica fine power had been subjected to a surface treatment with a surface treating agent having a negatively chargeable polar group such as a fluorine silane coupling agent, and a surface treating agent having a positively chargeable polar group such as an amine type silane coupling agent. This toner is useful as a toner for nonmagnetic mono-component development due to having a high charge level, and being superior in charge-rising property and flowability.
Furthermore, a toner to which a dry type silica fine powder which has a positively charged polar group introduced therein and which had been subjected to a hydrophobilization treatment using a hydrophobidizing agent, and a wet type silica fine powder which had been subjected to a hydrophobilization treatment with a silicone oil were externally added; and a toner to which a dry type silica fine powder having a positively charged polar group and a hydrophobic group, and a wet type silica fine powder having a positively charged polar group and a fluorine-containing polar group were externally added were proposed. These toners are superior in charge-rising property, durability, and environment stability.
However, since a silica fine powder is used as an inorganic fine powder in the foregoing variety of toners, even if the silica fine powder is subjected to a variety of surface treatment, it has been still difficult to maintain positively charging property of a toner, resulting from the strong negatively charging property exhibited by the silica fine powder. Therefore, according to the aforementioned various toners, in the case in which a toner is subjected to strong stress in a development device, such as a case in which agitation of a toner is continued in a development device by continuing printing at a low coverage for a long time period, and the like, positively charging property of the toner is likely to decrease, and fogging in a formed image and toner scattering from the development device are likely to occur.